A major science goal of future, large-aperture, optical space telescopes is to directly image and spectroscopically analyze reflected light from potentially habitable exoplanets. To accomplish this, the optical system must suppress diffracted light from the star to reveal point sources approximately ten orders of magnitude fainter than the host star at small angular separation. Coronagraphs with microdot apodizers achieve the theoretical performance needed to image Earth-like planets with a range of possible telescope designs, including those with obscured and segmented pupils. A test microdot apodizer with various bulk patterns (step functions, gradients, and sinusoids) and 4 different dot sizes (3 µm, 5 µm, 7 µm, and 10 µm) made of small chrome squares on anti-reflective glass was characterized with microscopy, optical laser interferometry, as well as transmission and reflectance measurements at wavelengths λ=600 nm and λ=800 nm. Microscopy revealed the microdots were fabricated to high precision. Results from laser interferometry showed that the phase shifts observed in reflection vary with the local microdot fill factor. This effect is not explained purely by interference between reflected fields from the chrome and glass portions. Transmission measurements showed that microdot fill factor and transmission were linearly related for dot sizes ≥5 µm. However, anomalously high transmittance was measured when the dot size is <5× the wavelength (i.e. ∼3 µm) and the fill factor is approximately 50%, where the microdot pattern becomes periodic. The transmission excess is not as prominent in the case of larger dot sizes suggesting that it is likely to be caused by the interaction between the incident field and electronic resonances in the surface of the metallic microdots. We used our empirical models of the microdot apodizers to optimize a second generation of reflective apodizer designs, specifically for demonstrating end-to-end instrumentation for planet characterization at Caltech's High Contrast Spectroscopy Testbed for Segmented Telescopes (HCST), and confirmed that the amplitude and phase of the reflected beam closely matches the ideal wavefront.
We discuss the use of parametric phase-diverse phase retrieval to characterize and optimize the transmitted wavefront of a high-contrast apodized pupil coronagraph with and without an apodizer. We apply our method to correct the transmitted wavefront of the HiCAT (High contrast imager for Complex Aperture Telescopes) coronagraphic testbed. This correction requires a series of calibration steps, which we describe. The correction improves the system wavefront from 16 nm RMS to 3.0 nm RMS for the case where a uniform circular aperture is in place. We further measure the wavefront with the apodizer in place to be 11.7 nm RMS. Improvement to the apodized pupil phase retrieval process is necessary before a correction based on this measurement can be applied.
The differential polarization visibilities R Q and R U of an object are the ratios of its visibilities corresponding to orthogonal polarizations, the interferometric analogs to Stokes Q and U intensity images. The measurement of differential polarization visibilitites can be used for constraining inner parts of circumstellar envelopes of young or evolved stars at the diffraction limited resolution of the feeding telescope. We demonstrate the estimation of both amplitude and phase of R Q and R U from data obtained using SCExAO VAMPIRES through the full pupil of the 8-m Subaru telescope using the differential speckle polarimetry technique. The correction for biases arising due to instrumental polarization effects is discussed. The accuracy of R Q and R U measurement with VAMPIRES is limited by imperfect knowledge of instrumental polarization and amounts to 5 × 10 −3 .
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